1. Field of the Invention
The present invention relates to an electrical device mounting wiring board comprising an insulation substrate with electrical devices such as capacitor and semiconductor device mounted on the surface or inside thereof, and a method of producing the same.
2. Description of Related Art
Recently electronics apparatuses have been becoming more compact, and the so-called mobile computing is becoming popular in which people carry hand-held information terminals or computers and operate them away from their homes or offices. To build such compact electronics apparatuses, demands have been growing for a multi-layer wiring board of compact and thin construction with fine patterning.
Semiconductor devices are mounted on the multi-layer wiring board by mounting a package having the semiconductor devices hermetically sealed therein on the multi-layer wiring board. However, this mounting method requires the step of electrically connecting the package with an electrical circuit layer provided on the surface of the multi-layer wiring board. As a result, the production process for completing an electrical apparatus that includes the semiconductor devices includes many steps, leading to a high production cost. Moreover, since the semiconductor devices can be mounted only on the surface of the multi-layer wiring board, there has been a limitation to the size and thickness reduction of the electrical apparatus and to the increase in the packaging density of the semiconductor devices.
Connection between the package and the wiring layer is made, for example, by wire bonding. In this case, connection pads provided on the wiring layer must be arranged at intervals of about 200 xcexcm. This requirement also hampers the progress of size reduction. Also because terminals of the package must be individually connected to the connection pads, it takes a long time to make connection resulting in low productivity. Furthermore, since the bonding wire connects the package terminal and the connection pad while winding in a loop shape, a clearance of about 0.5 mm must be secured in the direction of thickness of the package. This has also been hampering the progress of size and thickness reduction of the electrical apparatuses.
In order to solve these problems, module construction where a plurality of semiconductor devices are stacked is proposed in, for example, Japanese Unexamined Patent Publication (KOKAI) No. 9-186204 (1997), Japanese Unexamined Patent Publication (KOKAI) No. 9-283697 (1997) and Japanese Unexamined Patent Publication (KOKAI) No. 11-40745 (1999).
This construction makes it possible to reduce the total area of the wiring board. At the same time, however, there is a problem of increasing thickness of the electrical apparatus including the semiconductor devices as a whole. Moreover, a complex step is involved for connecting the stacked devices, thus resulting in an increasing production cost.
With the background described above, such attempts have been made as incorporating semiconductor devices in the wiring board itself. These attempts are roughly divided into techniques that are based on the conventional printed wiring board, and techniques that are based on the build-up wiring board.
The techniques based on the printed wiring board is disclosed, for example, in Japanese Unexamined Patent Publication (KOKAI) No. 8-88471 (1996). With the technique disclosed in this publication, a printed wiring board is made with part thereof being machined to form recesses. Semiconductor devices are housed in the recesses and electrically connected to wiring lines, via wires provided in the wiring board. Then another wiring board is laminated on the wiring board by thermal pressing or the like, thereby forming a multi-layer wiring board.
The techniques based on the build-up wiring board is disclosed, for example, in Japanese Unexamined Patent Publication (KOKAI) No. 9-321408 (1997) and Japanese Unexamined Patent Publication (KOKAI) No. 9-46049 (1997). With the technique disclosed in these publications, a printed wiring board is made with part thereof being machined to form recesses. Semiconductor devices are housed in the recesses and a circuit is formed thereon by build-up technique. Thus the semiconductor devices are incorporated in the wiring board.
However, any of these techniques have a problem in the applicability to mass production. With the technique based on the printed wiring board, the semiconductor device may be subjected to breakage during the thermal pressing step. This is because one of the boards being thermally pressed has already been cured and therefore even slight unevenness in the recesses that have been machined leads to stress concentration in the uneven portions. Also because shrinkage of the resin used for making the board due to curing proceeds differently on the front and back sides of the semiconductor device, there has been a problem of the semiconductor devices to break due also to the stress concentration caused by shrinkage during curing.
With the technique based on the build-up wiring board, on the other hand, stresses described above are less likely to be generated and lead to less occurrence of breakage of the semiconductor devices. However, there are other problems caused by the step of forming the wiring pattern, that is a wet process such as plating, and by the intrinsically highly moisture-absorption property of a photosensitive resin that is used in the build-up wiring board. Specifically, it has been difficult to put the build-up wiring board into commercial operation, because of insufficient heat resistance (causing swell and other defects generated during solder reflow) due to the absorption of moisture, and significant decrease in reliability demonstrated in thermal cycle test and pressure cooker test.
For the build-up wiring board, much attention has been paid to the laser via board wherein via holes are made by means of carbon dioxide gas laser. However, when it is attempted to incorporate the semiconductor devices in the laser via board, there may occur such a problem as the circuit over the semiconductor devices is broken by the heat generated by the laser when forming the via holes. Also because silicon used in the substrate of the semiconductor device has high heat conductivity, heat of laser is absorbed by the silicon substrate. Consequently, residue of resin, called the smear, deposit on the silicon substrate after machining with the laser. The resin residue hampers electrical connection. As a result, it has been also difficult to put the technique based on the laser via board into commercial operation
Problems similar to the above arise also in cases where electrical components other than semiconductor devices such as capacitor and resistor are mounted on the multi-layer wiring board.
First object of the present invention is to provide an electrical device mounting wiring board with electrical devices incorporated therein, thereby making it possible to decrease the size of electrical apparatuses and to increase the packaging density of component devices.
Second object of the present invention is to provide a method of producing the electrical device mounting wiring board that makes it possible to easily produce a wiring board having electrical devices incorporated inside thereof.
The present inventors have studied intensively to reduce the size of a wiring board incorporating electrical devices and to simplify a method of producing the same. As a result, it has been found that stress concentration on the electrical devices when the electrical devices are incorporated in the wiring board can be mitigated by forming a resin layer having a glass transition point lower than the curing temperature of a thermosetting resin in the wiring board which incorporates the electrical devices, between at least one of top and bottom surfaces of the electrical device and an insulation layer constituting the wiring board, thereby making it possible to drastically improve the productivity of the step of incorporating the electrical devices.
It has also been found that stress concentration can be mitigated further by arranging electrical device on or in an insulation sheet that includes a thermosetting resin that has not yet cured, and incorporating the electrical device simultaneously as the insulation sheet cures.
The electrical device is typically a semiconductor device with electrodes arranged in a matrix on one principal surface thereof.
The substrate of a semiconductor device is made of a very brittle substance such as single crystal silicon. Therefore, when incorporating semiconductor devices in a multi-layer wiring board that includes thermosetting resin in the insulation substrate thereof, it is preferable to fasten the semiconductor devices by insulation layers placed over and below thereof. In case there is a very large difference in the thermal expansion coefficient between the insulation layer of the wiring board containing an organic resin and the semiconductor device, there has been a high possibility of the semiconductor devices to break due to the stress generated when the wiring board is heated to cure and the stress generated by a pressure applied for bonding the layers.
According to the present invention, the insulation substrate is maintained in a soft state until the temperature is raised from the room temperature to the curing start temperature, in order to incorporate the electrical device in the insulation substrate that is not yet cured or in the B stage. As a consequence, the electrical device is not subject to stress concentration and integrity of the devices is maintained.
At the time when the thermosetting resin included in the insulation substrate is cured, glass transition point Tg of the resin layer bonded to one or both sides of the electrical device is lower than the thermal curing temperature. As a result, since the resin layer softens at the thermal curing temperature, the electrical device is always enclosed by a soft material during the period from the start to the end of curing of the multi-layer wiring board. Thus integrity of the electrical device is maintained. As a consequence, it is made possible to relieve the electronic device from additional stress since the softened resin layer absorbs the stress that is generated by the shrinkage of the insulation substrate during curing.
Owing to the effects described above, the electrical device can be incorporated in the multi-layer wiring board with a high yield of production. Since the electrical device can be incorporated in the wiring board as well as being mounted on the wiring board, it is made possible to reduce the size of the electrical apparatus further.
Meanwhile, the present inventors have reached such a finding as follows. That is, when electrodes of the electrical device are connected to via hole conductors or the connection pads in the wiring layer, and a low-melting point metal, of which melting point is lower than the curing temperature of the thermosetting resin included in the insulation substrate, is included in the via hole conductors or the connection pads, it is made possible to carry out the electrical connection between the electrical of the electrical device and the via hole conductors and the fabrication of the wiring board at the same time, and the electrical device can be easily incorporated in the multi-layer wiring board.
The above objects as well as other objects, features and effects of the present invention will become apparent from the description of preferred embodiments with reference to the accompanying drawings.